It is the common practice for public utilities and similar companies to use service and maintenance trucks provided with some type of boom structure supporting a workman support bucket or basket at the outer end thereof. These structures are usually referred to as "aerial towers" and are particularly useful in servicing electric power and telephone lines, overhead traffic lights, street lamps and the like, and these structures are provided with some type of leveling mechanism to maintain the workman support bucket in a vertical position during all of the various adjustable positions of the boom structure.
Many of these aerial towers employ a mechanical leveling system to maintain the workman support bucket in a vertical position. These mechanical leveling systems usually employ one or more closed loops of flexible elongate material, such as cables, sprocket chains or other link and lever systems. In the event a workman in the bucket is injured and needs to be removed immediately, it is the common practice to lower the bucket to a position as close to the ground as possible and remove the workman therefrom, while the bucket is still in a vertical position. This can be a cumbersome and time-consuming operation and may delay treatment of the workman under circumstances where the delay of treatment, even for a few minutes, can be fatal.
It is also known to provide a bucket leveling system which includes a pendulum-operated control valve to regulate hydraulic fluid supply to a leveling motor. Such a leveling system is disclosed in U.S. Pat. No. 3,590,948 and this patent also discloses a manually operated bypass valve which may be used to override the normal pendulum-operated leveling control valve to tilt or rotate the bucket to a horizontal position for facilitating the removal of an injured workman. This pendulum-operated leveling system includes a gravity-sensing weight supported on the lower end of a rod which is fixed at its upper end to a control valve adapted to control the flow of hydraulic fluid to a leveling fluid motor. This gravity-sensing weight is supported on the workman support bucket and is subject to being engaged by limbs and the like, particularly when working with the aerial tower in trees and the like, and the support rod may become bent or jammed to provide positioning of the bucket at other than the desired vertical position.